+Brooming Disease of Walnut+
A systemic brooming disease, observed on planted walnuts as early as 1917, has been the subject of considerable discussion during recent years, because it has now spread widely into the native black walnut growth. In 1932 Waite published that he had been observing the disease for some 15 years but that "it was unknown on the black walnut in the wild in this country or on planted trees away from the Japanese walnut." The disease has continued to increase in prevalence in recent years and is now widely distributed in native black walnut growth in Tennessee, Virginia, District of Columbia, Maryland, Delaware, and New Jersey. This extensive spread into the native growth during the last 15 or 20 years and the fact that reports indicate that all of the early cases of the disease were found near nursery-grown trees offer some evidence that the disease is an importation from another area or continent into the eastern black walnut zone. From the literature and oral reports, it seems that the disease is now present also in North Carolina, West Virginia, Pennsylvania, New York, Ohio, and Michigan. Surveys probably would uncover the disease among native wild and planted walnuts in other States.
[Footnote 11: Crandall, B. S., G. F. Gravatt, and M. M. Ryan. Root diseases of Castanea species and some coniferous and broadleaf nursery stocks, caused by Phytophthora cinnamomi. Phytopathology 35: 162-180. Illus. 1945.]
+Economic Importance and Hosts+
The black walnut is a valuable native forest tree, widely but not abundantly distributed in the eastern United States. It is extensively planted as a forest tree. The numerous plantings and natural stands around farm homes, along fences, and in pastures are also very valuable. More and more grafted ornamentals, and orchards of black walnut are being planted. For these the per-tree investment is high.
[Illustration: Figure 2.—The brooming disease of walnut. Severe brooming on Japanese walnut.]
The ultimate effect of the brooming disease on the black walnut is not known. Dr. Waite stated, "Trees even moderately attacked soon become worthless for nut production." Some affected black walnut trees, however, continue to produce small crops of nuts. Visible symptoms have been known to disappear. In addition, some seedlings, and probably large trees also, are infected without showing symptoms. Such observations indicate the complex nature of the disease. Detailed studies are needed, but at present this Division is not in position to do more than limited, part-time work on the disease.
The butternut, a widely distributed forest tree of minor importance, is seriously injured or killed by this disease. The disease severely damages or kills the Japanese walnut, which has been planted to a limited extent but is of little importance. According to Dr. Waite's report, the Persian, or English, walnut is attacked, but very few trees of this species are planted in the eastern States. Precautions should be taken to prevent the introduction of this disease into areas where it is not now present, particularly the western states.
Symptoms expressed by infected trees are viruslike, and Hutchins and Wester[12] were able to produce the brooming symptoms on a small number of trees by means of bark patch grafts, indicating that the brooming disease probably is caused by a virus.
[Illustration: Figure 3.—Brooming disease on black walnut. Ascending type, upright, sucker growth is typical of this species.]